I was just asking myself the same question this morning.... and within 5mins, I was in a long stream of traffic heading up a winding up a country hill and the traffic slowed from around 50mph to just under 40mph..... 6th felt to long and and lo and behold 5th was the choice: initially I box-shifted to 4th, it felt a bit too revvy (for meandering traffic) and next thing I was actually using 5th!!